Since his teenage years, Koji Hayashi has dreaded one thing: his stubborn, once-vivacious mother being hanged for murder after failing to win her long campaign for a retrial.

Left almost unchanged for a century, Japan's current retrial system is often labeled the "unopenable door" because the chances of being granted a legal do-over are so slim.

But hopes have grown of a change since a court last year overturned the wrongful conviction of the world's longest-serving death row prisoner Iwao Hakamata, whose case took 42 years to be reopened.